Theoretical models of adsorptive reactors which combine multibed pressure swing adsorption and chemical reaction have been studied for some limited types of reversible and irreversible reactions. A paper entitled, "Combined Reaction and Separation in Pressure Swing Processes," by E. Alpay et at. and presented at the International Symposium on Chemical Reaction Engineering, Sep. 25-28, 1994, Baltimore, Md., describes the advantages of such a system for a dissociation reaction producing two components where one of the products is the only adsorbing component. Another paper entitled, "A Theoretical Investigation of Pressure Swing Reaction," by N. F. Kirkby and J. E. P. Morgan, and published in the TRANSACTIONS OF INDUSTRIAL CHEMICAL ENGINEERING, Vol. 72, Part A, July 1994, explores a simplified model of pressure swing reaction applied to a non-adsorbable reactant undergoing an irreversible reaction to produce an adsorbable product. These studies indicate that combinations of pressure swing adsorption and reaction have some advantages over conventional reaction systems. The conclusions reached by these studies suggest that the results of the combined PSA and reaction process are significantly different from conventional PSA technology and the steps of the cyclic operation are dependent upon the many variables that relate to the relative adsorption of the products and reactants and the degree to which equilibrium reactions are affected by the adsorption of the reactants and the products of the reaction. To date there have been few commercial applications of combined pressure swing adsorption and reaction where the reactants and the products can be non-adsorbable, less-readily adsorbable and more-readily adsorbable.
Methanol is an important industrial chemical. Because of its simple molecular structure, it is used as a building block for larger, more complicated organic molecules. Traditionally, the primary uses of methanol were for chemical production as a feedstock or as a solvent. Two uses for methanol which will increase the demand for methanol are 1) the requirement for the addition of oxygenates to transportation fuels and 2) the potential use as an agricultural plant growth stimulant. Methanol is used in the manufacture of ethers such as MTBE, methyl tertiary butyl ether, a high octane gasoline blending component, whose increased demand coincides with the worldwide phasedown of lead in transportation fuels.
At present, methanol production is almost exclusively based on a direct hydrogenation of carbon monoxide according to the following equation: EQU CO+2H.sub.2 .revreaction.CH.sub.3 OH (1)
and if the reactants contain carbon dioxide, also: EQU CO.sub.2 +3H.sub.2 .revreaction.CH.sub.3 OH+H.sub.2 O (2)
To obtain reasonable reaction rates, (solid) catalysts have to be used. Important progress was noticed in this field in the late 1960's when highly active copper-based catalysts replaced zinc-based catalysts, allowing a reduction of the process pressure from 20-30 MPa and temperatures up to about 450.degree. C. to pressures ranging from 5-10 MPa and at considerably lower temperatures (200.degree.-280.degree. C.).
Methanol synthesis is a strongly exothermic (.DELTA.H.sub.298K =-91 KJ/mol) equilibrium limited reaction. Both reactions (1 and 2) are exothermic (release heat) as they proceed to methanol. An increase in the temperature unfavorably influences the position of the reaction equilibrium. Moreover, the copper-based catalysts lose their activity very quickly if their maximum allowable operating temperature has been exceeded due to insufficient heat removal. As a consequence, the reactions never reach completion in the reactor. As the reaction passes through the catalyst bed, the reaction slows down and even stops, approaching an equilibrium composition based on the reactor conditions of pressure and temperature. In addition, both reactions result in a decrease in the number of moles. Thus, methanol synthesis is favored by increasing pressure and decreasing temperature, and methanol synthesis is adversely affected by decreasing pressure and rising temperature.
Two types of reactors are presently used for the commercial methanol synthesis: adiabatic bed reactors and cooled tubular reactors. An adiabatic bed reactor consists of several fixed catalyst sections or beds in series. The temperature in the beds is controlled either with heat exchangers between the beds or by introducing cold synthesis gas between the catalyst beds. In the latter or "cold-shot" construction, no heat exchangers are needed in the reactor. The main disadvantage of the cold-shot system is the dilution of the product which increases the costs of separating the product from the unconverted reactor effluent.
A cooled tubular reactor e.g. consists of a bundle of tubes filled with the catalyst. The tubes are installed in a cooling jacket and cooled e.g. by boiling water so that steam is generated from the heat generated by the reaction. This construction makes the reactor temperature easy to control. A book entitled "Methanol Production and Use", edited by the Wu-Hsun Cheng and Harold H. Kung, published by Marcel Dekker, Inc., New York, 1994, describes the typical methanol production processes on pages 51-73 and particularly describes the thermodynamics and kinetics on pages 53-61. The above pages are herein incorporated by reference.
Due to the unfavorable position of the reaction equilibrium at process temperature, the methanol concentration in the non-condensable reactor effluent is low; i.e., the methanol molar fraction usually does not exceed a value of 0.1. Therefore, the separation of the reaction product by condensation causes essential difficulties, and the heat transfer coefficients are very low such that large heat exchange areas in the condenser are required. Because of the incomplete conversion, the unconverted reactants have to be recycled to the reactor inlet by means of a compressor. The recycle ratios are commonly in the range of 5 to 10 for methanol plants. Typically the recycle ratio for a particular plant will vary according to the operating pressure, the concentration of inert compounds in the feed to the reaction, and the design approach to equilibrium.
U.S. Pat. No. 4,968,722 to Westerterp discloses a process for producing methanol by reacting carbon monoxide and hydrogen wherein these reactants in the gas phase are introduced into a reaction zone comprising one or more fixed catalyst beds and a liquid absorbent. The liquid absorbent selectively absorbs substantially all of the methanol produced. The liquid absorbent is subsequently pumped out of the reactor and flashed to recover the product methanol. In an earlier patent, U.S. Pat. No. 4,731,387, Westenerp discloses a methanol reaction zone containing a fixed bed of coarse catalyst particles having interstices between them and passing a fine particle solid adsorbent downwardly through the interstices to adsorb substantially all of the methanol product. U.S. Pat. No. 5,254,368 to Kadlec et al. discloses the integral coupling of reaction with a single-bed rapid cycle pressure swing adsorber to provide better separation and more efficient, irreversible reactions wherein the reactant is adsorbed, and those wherein the reactant is not adsorbed. Kadlec et al. describe the use of a single-bed pressure-periodic process for a two-reactant CO oxidation process for automobile pollution control. Kadlec et al. further teach a sequence of operation of the single-bed process which includes a delay step following the introduction of the feed gas and prior to an exhaust step, such that during the delay step the pressure within the single-bed adsorber is permitted to equalize as a continuous stream of product is removed.
Pressure swing adsorption (PSA) provides an efficient and economical means for separating a multi-component gas stream containing at least two gases having different adsorption characteristics. The more strongly adsorbable gas can be an impurity which is removed from the less strongly adsorbable gas which is taken off as product, or the more strongly adsorbable gas can be the desired product which is separated from the less strongly adsorbable gas. For example, it may be desired to remove impurities such as carbon monoxide and light hydrocarbons from a hydrogen-containing feed stream to produce a purified (99+%) hydrogen stream for use in a downstream catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more strongly adsorbable gases, such as ethane, from a feedstream to produce an ethane-rich product.
In pressure swing adsorption, a multi-component gas stream is typically fed to at least one of a plurality of adsorption zones at an elevated pressure effective to adsorb at least one component, while at least one other component passes through. At a defined time, the feedstream to the adsorber is terminated and the adsorption zone is depressurized by one or more cocurrent depressurization steps wherein pressure is reduced to a defined level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. Then, the adsorption zone is depressurized by a countercurrent depressurization step wherein the pressure on the adsorption zone is further reduced by withdrawing desorbed gas countercurrently to the direction of the feedstream. Finally, the adsorption zone is purged and repressurized. The combined gas stream produced during the countercurrent depressurization step and the purge step is typically referred to as the tail gas stream. The final stage of repressurization is typically performed by introducing a slipstream of product gas comprising the lightest gas component produced during the adsorption step. This final stage of repressurization is often referred to as product repressurization.
In multi-zone systems there are typically additional steps, and those noted above may be done in stages. U.S. Pat. Nos. 3,176,444 issued to Kiyonaga, 3,986,849 issued to Fuderer et al., and 3,430,418 and 3,703,068 both issued to Wagner, among others, describe multi-zone, adiabatic pressure swing adsorption systems employing both cocurrent and countercurrent depressurization. The disclosures of these patents are incorporated by reference in their entireties.
Various classes of adsorbents are known to be suitable for use in PSA systems, the selection of which is dependent upon the feedstream components and other factors generally known to those skilled in the art. In general, suitable adsorbents include molecular sieves, silica gel, activated carbon, and activated alumina. When PSA processes are used to purify hydrogen-containing streams, the hydrogen is essentially not adsorbed on the adsorbent.
A British patent application GB 2,233,329A, published Jan. 9, 1991 discloses a process for the production of methanol wherein synthesis gas is produced by steam reforming of natural gas and secondary reforming using air and wherein the synthesis gas is compressed into a conventional methanol loop comprising a first methanol synthesis reactor in which nitrogen is allowed to build up in the recycle stream. A large purge is taken from the recycle stream and passed to a second methanol synthesis reaction system containing a mixture of methanol synthesis catalyst and a methanol adsorbent wherein further methanol is produced. The second methanol synthesis reaction system operates cyclically, recovering methanol by sweeping methanol from the adsorbent with desulfurized natural gas or depressurizing. The methanol product is recovered by condensing the desorbed methanol.
Improved processes are sought for the combination of pressure swing adsorption and reaction for reversible reactions wherein non-adsorbable and less-readily adsorbable reactants react to produce adsorbable products.
Improved processes are sought for the production of methanol from synthesis gas.
Processes are sought which extend the equilibrium conversion and provide a methanol product with fewer impurities.
Processes which produce methanol have been limited by the degree to which equilibrium conversion in reversible reactions can be approached. Prior attempts to employ adsorbents and reaction have been limited by the tendency of the reactions to reverse upon depressurization or desorption of the methanol resulting in the loss of conversion and lower product purity. Processes for the production of methanol are sought which permit the production of methanol to proceed with a greater conversion per pass and with a yield of a high purity product. In addition, processes are sought which substantially reduce or eliminate the recycle of unreacted components.